1. Field of the Invention
The present invention relates in general to turbine rotors and particularly concerns the mounting of nonmetallic rotor blades having airfoil shaped dovetails to a rotor disk via a plurality of circumferentially spaced metal platform members having rotor blade support surfaces corresponding to the airfoil shaped dovetails of the rotor blades.
2. Description of Prior Developments
To improve the performance of turbines, new rotor blade materials have been developed. Such materials include both metals and nonmetallics. Nonmetallics, such as carbon/carbon and ceramics are lighter than metal and require little or no cooling. Unfortunately, most high temperature nonmetallic materials like carbon/carbon and ceramics do not have the bending capabilities of metal.
The inability to withstand significant bending loads presents a design problem insofar as the configuration of nonmetallic rotor blades is concerned. More particularly, rotor blades usually have a platform that forms the inner flowpath of the gas stream. For example, as seen in FIGS. 1 and 2, a metal rotor blade 10 includes a platform 12 which extends circumferentially outward in a cantilevered fashion on each side of the airfoil root section 14 of airfoil 15. When rotated during use, the platforms 12 are subjected to centrifugal bending loads as well as bending loads from the motive exhaust gases.
Metal platforms can be designed to withstand these bending loads but nonmetallic platforms of materials like carbon/carbon and ceramics have generally been considered incapable of reliably sustaining such loads. This has resulted in the use of metallic materials for the platforms. A previous attempt to solve the platform bending and loading problem involved removing the nonmetallic platform from the nonmetallic blade and replacing it with a metal platform.
As seen in FIGS. 3 through 6, a separate metal platform 16 was created to replace the integral nonmetallic platform 12 previously formed homogeneously with prior rotor blade designs of the type depicted in FIGS. 1 and 2. The metal platform 16 was equipped with forward and aft integral legs 18, 20 with a dovetail 22 formed on each leg. The dovetails 22 on each leg 18, 20 fit into the same disk dovetail slot 24 (FIGS. 5 and 6) as the rotor blade 10.
The platform 16 included an airfoil shaped hole 26 sized larger than the blade airfoil root section 14 to accommodate assembly of the platform 16 over the nonmetallic airfoil 30. This oversizing was required because the blade airfoil tip section 32 (FIG. 5) is typically larger in places than the root section 14.
The platform 16 was installed over the blade airfoil tip 32 and lowered down to the airfoil root 14. Next, the blade-platform assembly was inserted into and secured within the disk dovetail slot 24 via blade dovetails 33 and platform dovetails 22. Finally, as seen in FIG. 5, the forward then the aft blade seals and retainers 34, 36 were installed on the rotor disk 38.
A significant problem associated with using the separate metal platform 16 on the nonmetallic airfoil 30 of the type noted above is the excessive loss of precious cooling air 39 which spills out of the assembly clearance gap 40 defined between the airfoil root section 14 and the airfoil shaped hole 26 in the platform 16. This leakage is best seen in FIGS. 4 and 5. The cooling air 39 also leaks out between adjacent platform edges 42 at the flowpath surface 44 (FIGS. 5 & 6) and between the forward and aft legs 18, 20.
Another problem encountered with the use of the separate metal platform 16 is excessive bending experienced by its unsupported central portion 45. That is, the platform 16 bends at its center because it is only supported by the forward and aft legs 18, 20.
Referring again to FIGS. 1 and 2, another area, other than the blade platforms, where bending stress presents a significant design problem is in the blade shank area 46 through which the airfoil root 14 transitions into a straight dovetail neck 48. Critical high stress areas are located at the leading and trailing edges 50, 52 where the airfoil blade 15 extends circumferentially beyond the straight dovetail neck 48 creating a large offset angle 54. The larger the offset angle 54, the greater the bending load in the shank area 46. Even with a small offset angle, the resulting stress levels have been found unacceptable for nonmetallic materials like carbon/carbon and ceramics.
In order to improve the shank bending problem and loading problem associated with the design of FIGS. 1 and 2, two changes to the configuration of rotor blade 10 were made as shown in FIGS. 7 and 8. First, a costly curved dovetail 56 was introduced to help reduce the offset angle 54 in the shank area 46 adjacent the straight dovetail 58 of FIG. 1.
Next, the airfoil 15 was changed from a high camber shape to a low chamber shape. This reduction in camber also helped to reduce the offset angle 54 in the shank 46. Unfortunately, by changing the airfoil 15 from a high camber profile to a low camber profile, a significant loss in performance results.
Still another problem associated with the use of nonmetallic rotor blades having curved dovetails and curved dovetail necks 62 is the width of the disk dovetail post 60 (FIG. 6) which is, by necessity, extremely thin at the trailing edge 52. This thin section experiences relatively high stress levels during engine operation. Such stress can result in reduced life of the rotor disk.
A thin dovetail post is required because a carbon/carbon or ceramic blade will only work satisfactorily with a large single tang dovetail which is wider than conventional multiple tang or "fir tree" dovetails. Moreover, the nonmetallic airfoil 15 must transition into a relatively large dovetail neck 62 which provides the required support between the airfoil and the curved dovetail 56. If possible, the resulting thin dovetail post should be avoided.
Accordingly, a need exists for a rotor blade mounting assembly which avoids the problems associated with conventional metallic blade platforms and which readily accommodates the working stress levels present in modern gas turbine engine rotor blades.